regrename: Fix -fcompare-debug issue in find_rename_reg [PR103756]

The r12-5978 change caused a -fcompare-debug issue, because without
-g a chain might start with a noop move, but with -g there could be
one or more DEBUG_INSNs in the chain before the noop move and so
regrename could make different decisions between -g and -g0.

Note, I must say I don't really understand the original change much,
if we want to make sure the noop moves are removed, couldn't regrename
during building of those du chains simply remove the noop moves instead?

2021-12-30  Jakub Jelinek  <jakub@redhat.com>

	PR rtl-optimization/103756
	* regrename.c (find_rename_reg): Test noop_move_p on the first
	non-debug insn in the chain rather than on the first insn.

	* g++.dg/opt/pr103756.C: New test.

diff --git a/gcc/regrename.c b/gcc/regrename.c
index fe72fcc362413e2eaefe8f5fc377464395e338aa..c34ea6be76b36e56df58c814d747a03f2f2b277a 100644 (file)
--- a/gcc/regrename.c
+++ b/gcc/regrename.c
@@ -394,10 +394,15 @@ find_rename_reg (du_head_p this_head, enum reg_class super_class,
                          this_head, *unavailable))
     return this_head->tied_chain->regno;
 
-  /* If this insn is a noop move, then do not rename in this chain as doing so
-     would inhibit removal of the noop move.  */
-  if (noop_move_p (this_head->first->insn))
-    return best_new_reg;
+  /* If the first non-debug insn is a noop move, then do not rename in this
+     chain as doing so would inhibit removal of the noop move.  */
+  for (struct du_chain *tmp = this_head->first; tmp; tmp = tmp->next_use)
+    if (DEBUG_INSN_P (tmp->insn))
+      continue;
+    else if (noop_move_p (tmp->insn))
+      return best_new_reg;
+    else
+      break;
 
   /* If PREFERRED_CLASS is not NO_REGS, we iterate in the first pass
      over registers that belong to PREFERRED_CLASS and try to find the

diff --git a/gcc/testsuite/g++.dg/opt/pr103756.C b/gcc/testsuite/g++.dg/opt/pr103756.C
new file mode 100644 (file)
index 0000000..d6d7596
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/pr103756.C
@@ -0,0 +1,57 @@
+// PR rtl-optimization/103756
+// { dg-do compile }
+// { dg-options "-std=c++17 -O -fcompare-debug -fconserve-stack -frename-registers -fno-tree-ch -fira-algorithm=priority" }
+
+char __copy_move_b___trans_tmp_9;
+template <typename> struct __iterator_traits;
+template <typename _Tp> struct __iterator_traits<_Tp *> {
+  typedef _Tp &reference;
+};
+template <typename _Iterator> struct reverse_iterator {
+  _Iterator current;
+  reverse_iterator();
+  reverse_iterator(reverse_iterator &__x) : current(__x.current) {}
+  _Iterator base() { return current; }
+  typename __iterator_traits<_Iterator>::reference operator*() {
+    return *current;
+  }
+  reverse_iterator operator--() {
+    ++current;
+    return *this;
+  }
+};
+template <typename _IteratorL, typename _IteratorR>
+auto operator-(_IteratorL __x, _IteratorR __y) {
+  return __y - __x.base();
+}
+struct __copy_move_backward {
+  template <typename _BI1, typename _BI2>
+  static _BI2 __copy_move_b(_BI1 __first, _BI1 __last, _BI2 __result) {
+    typename __n = __last - __first;
+    for (; __n > 0; --__n) {
+      reverse_iterator __trans_tmp_8 = --__result;
+      *__trans_tmp_8 = __copy_move_b___trans_tmp_9;
+    }
+    return __result;
+  }
+};
+template <int, typename _BI1, typename _BI2>
+inline _BI2 __copy_move_backward_a2(_BI1 __first, _BI1 __last, _BI2 __result) {
+  return __copy_move_backward::__copy_move_b(__first, __last, __result);
+}
+template <int _IsMove, typename _BI1, typename _BI2>
+_BI2 __copy_move_backward_a1(_BI1 __last, _BI2 __result) {
+  _BI1 __first;
+  return __copy_move_backward_a2<_IsMove>(__first, __last, __result);
+}
+template <int _IsMove, typename _II, typename _OI>
+void __copy_move_backward_a(_II, _OI __result) {
+  reverse_iterator<unsigned char *> __trans_tmp_7 =
+      __copy_move_backward_a1<_IsMove>(__trans_tmp_7, __result);
+}
+template <typename _BI1, typename _BI2>
+void move_backward(_BI1 __first, _BI2 __result) {
+  __copy_move_backward_a<true>(__first, __result);
+}
+reverse_iterator<unsigned char *> __rotate___first;
+void __rotate() { move_backward(__rotate___first, __rotate___first); }